fix(responder): preserve custom Content-Type header in Data

responder.go

@@ -70,9 +70,11 @@ func PlainText(w http.ResponseWriter, r *http.Request, v string) {
 }
 
 // Data writes raw bytes to the response, setting the Content-Type as
-// application/octet-stream.
+// application/octet-stream if not already set.
 func Data(w http.ResponseWriter, r *http.Request, v []byte) {
-	w.Header().Set("Content-Type", "application/octet-stream")
+	if w.Header().Get("Content-Type") == "" {
+		w.Header().Set("Content-Type", "application/octet-stream")
+	}
 	if status, ok := r.Context().Value(StatusCtxKey).(int); ok {
 		w.WriteHeader(status)
 	}

responder_test.go

@@ -0,0 +1,108 @@
+package render
+
+import (
+	"bytes"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestData_DefaultContentType(t *testing.T) {
+	w := httptest.NewRecorder()
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+	payload := []byte("binary data")
+
+	Data(w, r, payload)
+
+	if got := w.Header().Get("Content-Type"); got != "application/octet-stream" {
+		t.Fatalf("expected Content-Type application/octet-stream, got %q", got)
+	}
+
+	if !bytes.Equal(w.Body.Bytes(), payload) {
+		t.Fatalf("expected body %q, got %q", payload, w.Body.Bytes())
+	}
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("expected status code %d, got %d", http.StatusOK, w.Code)
+	}
+}
+
+func TestData_CustomContentType(t *testing.T) {
+	tests := []struct {
+		name        string
+		contentType string
+		payload     []byte
+	}{
+		{
+			name:        "image/png",
+			contentType: "image/png",
+			payload:     []byte{0x89, 'P', 'N', 'G', 0x0D, 0x0A, 0x1A, 0x0A},
+		},
+		{
+			name:        "application/pdf",
+			contentType: "application/pdf",
+			payload:     []byte("%PDF-1.4 sample pdf content"),
+		},
+		{
+			name:        "custom text/csv with charset",
+			contentType: "text/csv; charset=utf-8",
+			payload:     []byte("id,name\n1,test\n"),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			w := httptest.NewRecorder()
+			r := httptest.NewRequest(http.MethodGet, "/", nil)
+
+			w.Header().Set("Content-Type", tt.contentType)
+			Data(w, r, tt.payload)
+
+			if got := w.Header().Get("Content-Type"); got != tt.contentType {
+				t.Fatalf("expected Content-Type %q, got %q", tt.contentType, got)
+			}
+
+			if !bytes.Equal(w.Body.Bytes(), tt.payload) {
+				t.Fatalf("expected body %q, got %q", tt.payload, w.Body.Bytes())
+			}
+		})
+	}
+}
+
+func TestData_WithStatus(t *testing.T) {
+	w := httptest.NewRecorder()
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+	Status(r, http.StatusAccepted)
+
+	w.Header().Set("Content-Type", "application/zip")
+	payload := []byte("zipdata")
+
+	Data(w, r, payload)
+
+	if w.Code != http.StatusAccepted {
+		t.Fatalf("expected status code %d, got %d", http.StatusAccepted, w.Code)
+	}
+
+	if got := w.Header().Get("Content-Type"); got != "application/zip" {
+		t.Fatalf("expected Content-Type application/zip, got %q", got)
+	}
+
+	if !bytes.Equal(w.Body.Bytes(), payload) {
+		t.Fatalf("expected body %q, got %q", payload, w.Body.Bytes())
+	}
+}
+
+func TestData_EmptyPayload(t *testing.T) {
+	w := httptest.NewRecorder()
+	r := httptest.NewRequest(http.MethodGet, "/", nil)
+
+	Data(w, r, []byte{})
+
+	if got := w.Header().Get("Content-Type"); got != "application/octet-stream" {
+		t.Fatalf("expected Content-Type application/octet-stream, got %q", got)
+	}
+
+	if len(w.Body.Bytes()) != 0 {
+		t.Fatalf("expected empty body, got %q", w.Body.Bytes())
+	}
+}